How to shift letters to the right or left? - c

Question

I am given a file with a string, for example "The United States was founded in *1776*". What I cannot figure out is how to shift letters one space to the left or right and have the letters wrap around. I am able to shift the letters from an a to b but not change its location within the word.
Example of this output would be:
"heT
nitedU
tatesS
asw
oundedf
ni
1776**"

Answer 1

In C, strings are stored as an array of chars in memory. Unlike C++ vectors, you can not insert or remove element within the array, you can only access their value or change their value.
If you declare a C string as follows:
char *myStr = "Fred";
It will be stored in memory as a five character array with the 5th character being the zero value which terminates a C string:
myStr[0] = 'F'
myStr[1] = 'r'
myStr[2] = 'e'
myStr[3] = 'd'
myStr[4] = 0
You need to design a for loop that copies each array element to the one before, while remembering that you need to save the one you are about to overwrite. In this example, it should result in the following copy operations being performed:
len = strlen(myStr);
saveCh = myStr[0];
myStr[0] = myStr[1];
myStr[1] = myStr[2];
myStr[2] = myStr[3];
myStr[3] = saveCh;
So now your job is to create a for loop that does that for any C string of any length.
So to rotate the chars within a C string to the left, you need to copy each char in the array at index i to previous array element i-1. The tricky part is to handle the wrap around properly when i=0 (in this example, you want to copy myStr[0] to myStr[3]. Now do that with a for loop.
You need to also understand that the last character of any C string is the null character (value zero), which terminates a C string. If you modify that element in the array, then your string will break. That is why saveCh is copied to myStr[3] and not to myStr[4].
void rotateStrLeftOneChar(char *myStr) {
// Always check for error and special cases first!
// If myStr is a NULL pointer, do nothing and exit
// If myStr is less than 2 chars, nothing needs to be done too.
if ((myStr != NULL) && (strlen(myStr)>1)) {
int len = strlen(myStr);
char saveCh = myStr[0];
int i = 0;
// Copy each char at index i+1 left to index i in the array
for(i=0; i<len-2; i++)
myStr[i] = myStr[i+1];
// The last character is special and is set to saveCh
myStr[len-1] = saveCh;
}
}

Answer 2

If you just need to output the letters to shift to the left and don't want to change original input then you can do:
#include <stdio.h>
#include <string.h>
void shiftletters(char * input, int i);
int main () {
char input[256];
int shift;
printf("Enter input : ");
scanf("%[^\n]s", input);
printf("Number of shifts : ");
scanf("%d", &shift);
shiftletters(input, shift);
return 0;
}
void shiftletters(char * input, int numshifts)
{
char str[256] = {'\0'};
char * delim = " \t";
char * pch = NULL;
int j, k, len, shifts;
if (input == NULL)
{
printf ("Invalid input\n");
return;
}
strcpy (str, input);
pch = strtok (str, delim);
while (pch != NULL)
{
len = strlen (pch);
if ((numshifts == len) || (len == 1))
{
printf ("%s\n", pch);
pch = strtok (NULL, delim);
continue;
}
if (len < numshifts)
shifts = numshifts % len;
else
shifts = numshifts;
for(j=shifts; j<len; j++)
printf("%c", pch[j]);
for(k=0; k<shifts; k++)
printf("%c", pch[k]);
printf("\n");
pch = strtok (NULL, delim);
}
}
The output of the program:
Enter input : The United States was founded in *1776*
Number of shifts : 1
heT
nitedU
tatesS
asw
oundedf
ni
1776**

Answer 3

like this
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
enum { L = -1, R = 1};
char *rotate(char word[], int dir){
size_t len = strlen(word);
char *temp = malloc(len + 1);
if(!temp){
perror("malloc");
exit(EXIT_FAILURE);
}
strcpy(temp, word);
for(char *src = temp; *src; ++src, ++dir){//or use memmove
word[(dir+len)%len] = *src;
}
free(temp);
return word;
}
int main(int argc, char *argv[]) {
FILE *fp = fopen("data.txt", "r");
if(fp == NULL){
perror("fopen");
exit(EXIT_FAILURE);
}
if(argc < 2){
fprintf(stderr, "Usage %s L|R...\n", argv[0]);
exit(EXIT_FAILURE);
}
char word[64];
while(fscanf(fp, "%63s", word)==1){
for(char *shift = argv[1]; *shift; ++shift){
int dir = *shift == 'L' ? L : R;
rotate(word, dir);
}
printf("%s\n", word);
}
fclose(fp);
}
using memmove version
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
enum { L = -1, R = 1};
char *rotate1(char word[], int dir){
size_t len = strlen(word);
if(len > 2){
char temp;
if(dir == L){
temp = word[0];
memmove(word, word+1, len-1);
word[len-1] = temp;
} else if(dir == R){
temp = word[len-1];
memmove(word+1, word, len-1);
word[0] = temp;
}
}
return word;
}
int main(int argc, char *argv[]) {
FILE *fp = fopen("data.txt", "r");
if(fp == NULL){
perror("fopen");
exit(EXIT_FAILURE);
}
if(argc < 2){
fprintf(stderr, "Usage %s L|R...\n", argv[0]);
exit(EXIT_FAILURE);
}
char word[64];
while(fscanf(fp, "%63s", word)==1){
for(char *shift = argv[1]; *shift; ++shift){
int dir = *shift == 'L' ? L : R;
rotate1(word, dir);
}
printf("%s\n", word);
}
}